1. Technical Field
The present invention relates to optical lithography systems. More particularly, this invention relates to a method and system for correcting line width deviations.
2. Background Art
Since the beginning of the computer era, manufacturers in the field of integrated circuits (IC's) have sought to reduce the geometric size of the devices (e.g., transistors or polygates) present on integrated circuits. The benefits achieved in reducing device dimensions include higher performance and smaller packaging sizes. However, numerous limitations arise as manufacturers attempt to achieve smaller and smaller device sizes. One primary problem manufacturers face is that as smaller devices are sought, the precision required from the tools used to create IC devices increases beyond their capabilities. Given the prohibitive costs involved and potential time lost to wait for the next generation of tools, manufacturers are forced to find techniques and methods that will allow such tools to operate beyond their intended specifications.
One of the first steps in manufacturing integrated circuit chips involves the laying out or designing of the circuits to be packaged on a chip. Most integrated circuits are designed using computer aided design (CAD) layout tools. CAD tools allow chip manufactures to plan the layout of the circuits on a computer where they can be analyzed and stored. Once this step is finished, the designs need to be transferred onto the chip. Unfortunately, present day chip manufacturing tools often lack the capability to create increasingly complicated and compact integrated circuit chips.
The predominate method of creating devices on integrated circuit chips involves the use of masks. In general, masks typically comprise a transparent substrate on which various "circuit" patterns, determined by a CAD system, are disposed. That "circuit" pattern is then transferred onto the surface of a silicon wafer. The transfer of the pattern from the mask to the silicon substrate is accomplished by passing visible, ultraviolet, or even x-ray radiation (e.g., light) through the mask and onto a silicon substrate containing a photoresist material. Because the mask contains a pattern made up of solid lines and clear space, only those areas made up of clear space will allow radiation to pass. This process results in the creation of devices on the silicon substrate. This methodology is referred to as photolithography.
A popular method of creating mask patterns involves the use of chrome and is often referred to as chrome on glass (COG). It is recognized however that the methods and systems described herein are equally applicable to all masks that involve light blocking materials and/or attenuated mask systems. In attenuated mask devices, such as attenuated phase shifters and alternating phase shifters, the chrome or other light blocking material is replaced with an attenuating material that allows a small amount of light (e.g., 6%) to pass through and get phase shifted. The materials may include silicon nitride, carbon, thin chrome with an oxide, thin chrome with the clear areas etched, etc.
Unfortunately, the efficacy of all lithography tools is limited by numerous factors, and is especially limited by the resolution of the lens used to direct the radiation through the mask. When a system is being used within its resolution limits, an aerial image of the circuit will be printed onto the chip as desired (i.e., "on size"). However, when the tool is being used aggressively, that is, past the design limits of the tool, certain images will print with a deviation from their desired size. This is referred to as operating in a nonlinear regime. Thus, under certain circumstances, it is not unusual to have polygates deviate from their desired size by as much as 50 nanometers (nm), which is unacceptably high.
Lens resolution is a function of several factors that make up the "exposure system" and is typically expressed as: ##EQU1## where r is the resolution desired, .lambda. is the exposing wavelength, and Na is the numerical aperture of the lithography tool. For many of today's applications, the lithography tool is being used aggressively, usually past the design limits of the tool, when k.sub.1 is below 0.8. As noted, aggressive use of the tool is driven by the desire to create smaller and smaller devices.
When the tool is being used beyond its design limits, a specific line/space combination or "control grating" may be chosen to print on size and the tool is calibrated accordingly. However, because the tool is operating in a nonlinear manner, the other lines will print with a deviation from their desired size. Thus, without taking some further corrective measure, it is impossible to achieve a complete circuit pattern without significant line deviation.
There have been numerous attempts at solving this problem including those involving proximity corrections. Proximity correction methods work by modifying the dimensions of the chrome lines on the mask to compensate for the error caused by nonlinear operation. Thus, under this technique, it may be necessary to put a chrome line with a width of 0.95 microns on the mask to print a line with a width of 1.0 microns. However, because a given mask may contain millions of lines of varying dimensions, difficulties arise in providing an efficient and accurate method for calculating line modifications.
There exist several vendors who provide various product solutions utilizing proximity correction methods. One such product is FAIM, which requires the running of an aerial image simulation of the entire chip, determining the deviation of each line, and then correcting for each deviation. An aerial image (AI) simulation is a known method of using a computer system to generate predicted printed line deviations based upon various parameters, including the type of lithography tool. The methodology of correcting for each deviation under such systems typically involves adjusting the original chrome-on-glass data by the opposite amount of the aerial image deviation. For instance, if the AI shows an undesired edge movement of +100 nm, then the system would compensate by changing the input shape by -100 nm. Unfortunately, the input shape with the adjustment may have a different deviation than the original shape showed, so this technique would have to undergo an iterative process until the desired AI is achieved. Moreover, there is no guarantee of convergence. The result is that the run time to perform this operation can be prohibitively long.
Another product, PRECIM, requires the printing of a wafer with a predetermined pattern, measuring the pattern, and then running the results through a software package to come up with a set of convolution functions which can then be applied to the design data. Yet another vendor, Trans Vector Technologies, utilizes a rules-based system derived from the lithographic exposure system being used. Their system utilizes the width of the line, the distance to the nearest line, and the width of the nearest line.
In addition, various patents exist that address the same issue. For example, U.S. Pat. No. 5,208,124, entitled "Method of Making a Mask for Proximity Effect Correction in Projection Lithography," issued on May 4, 1993, to Sporon-Fiedler et al. teaches a method of adjusting line sizes on the glass mask based on a series of equations. Unfortunately, these equations are generic to all exposure systems and desired patterns and, therefore, fail to provide accurate line width corrections for all possible cases.
While these and numerous other techniques exist for line proximity correction techniques, none exist that take into account for the particular exposure system and circuit type being designed, and require only information regarding line width and line spacing. Thus, a need exists to provide an accurate and precise proximity correction technique for performing line width corrections during an optical lithography operation. The aforementioned art is herein incorporated by reference.